This invention relates to plasma doping systems used for ion implantation of workpieces and, more particularly, to methods and apparatus for measuring the ion dose implanted into the workpiece in plasma doping systems.
Ion implantation is a standard technique for introducing conductivity-altering impurities into semiconductor wafers. In a conventional ion implantation system, a desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity.
In some applications, it is necessary to form shallow junctions in a semiconductor wafer, where the impurity material is confined to a region near the surface of the wafer. In these applications, the high energy acceleration and the related beam forming hardware of conventional ion implanters are unnecessary. Accordingly, it has been proposed to use plasma doping systems for forming shallow junctions in semiconductor wafers. In a plasma doping system, a semiconductor wafer is placed on a conductive platen which functions as a cathode. An ionizable gas containing the desired dopant material is introduced into the chamber, and a high voltage pulse is applied between the platen and an anode or the chamber walls, causing formation of a plasma having a plasma sheath in the vicinity of the wafer. The applied voltage causes ions in the plasma to cross the plasma sheath and to be implanted into the wafer. The depth of implantation is related to the voltage applied between the wafer and the anode. A plasma doping system is described in U.S. Pat. No. 5,354,381 issued Oct. 11, 1994 to Sheng.
In the plasma doping system described above, the high voltage pulse generates a plasma and accelerates positive ions from the plasma toward the wafer. In other types of plasma systems, known as plasma immersion systems, a continuous RF voltage is applied between the platen and the anode, thus producing a continuous plasma. At intervals, a high voltage pulse is applied between the platen and the anode, causing positive ions in the plasma to be accelerated toward the wafer.
Exacting requirements are placed on semiconductor fabrication processes involving ion implantation with respect to the cumulative ion dose implanted into the wafer and dose uniformity across the wafer surface. The implanted dose determines the electrical activity of the implanted region, while dose uniformity is required to ensure that all devices on the semiconductor wafer have operating characteristics within specified limits.
One prior art approach to dose measurement in plasma doping systems involves measurement of the current delivered to the plasma by the high voltage pulses, as described in the aforementioned U.S. Pat. No. 5,354,381. However, this approach is subject to inaccuracies. The measured current includes electrons generated during ion implantation and excludes neutral molecules that are implanted into the workpiece, even though these neutral molecules contribute to the total dose. Furthermore, since the measured current passes through the wafer being implanted, it is dependent on the characteristics of the wafer, which may produce errors in the measured current. Those characteristics include emissivity, local charging, gas emission from photoresist on the wafer, etc. Thus, different wafers give different measured currents for the same ion dose. In addition, the measured current pulses include large capacitive or displacement current components which may introduce errors in the measurement.
A technique for plasma doping dosimetry is described by E. Jones et al. in IEEE Transactions on Plasma Science, Vol. 25, No. 1, February 1997, pp. 42-52. Measurements of implanter current and implant voltage are used to determine an implant profile for a single implant pulse. The implant profile for a single pulse is used to project the final implant profile and total implanted dose. This approach is also subject to inaccuracies, due in part to the fact that it depends on power supply and gas control stability to ensure repeatability. Furthermore, the empirical approach is time consuming and expensive.
In conventional ion implantation systems which involve the application of a high energy beam to the wafer, cumulative ion dose is typically measured by a Faraday cup, or Faraday cage, positioned in front of the target wafer. The Faraday cage is typically a conductive enclosure, often with the wafer positioned at the downstream end of the enclosure and constituting part of the Faraday system. The ion beam passes through the Faraday cage to the wafer and produces an electrical current in the Faraday. The Faraday current is supplied to an electronic dose processor, which integrates the current with respect to time to determine the total ion dosage. The dose processor may be part of a feedback loop that is used to control the ion implanter.
Various Faraday cage configurations for ion implanters have been disclosed in the prior art. Faraday cages positioned in front of semiconductor wafers are disclosed in U.S. Pat. No. 4,135,097 issued Jan. 16, 1979 to Forneris et al; U.S. Pat. No. 4,433,247 issued Feb. 21, 1984 to Turner; U.S. Robertson et al; U.S. Pat. No. 4,463,255 issued Jul. 31, 1984 to Robertson et al; U.S. Pat. No. 4,361,762 issued Nov. 30, 1982 to Douglas; U.S. Pat. No. 4,786,814 issued Nov. 22, 1988 to Kolondra et al; and U.S. Pat. No. 4,595,837 issued Jun. 17, 1986 to Wu et al. Faraday cages positioned behind a rotating disk are disclosed in U.S. Pat. No. 4,228,358 issued Oct. 14, 1980 to Ryding; U.S. Pat. No. 4,234,797 issued Nov. 18, 1980 to Ryding; and U.S. Pat. No. 4,587,433 issued May 6, 1986 to Farley.
Dose and dose uniformity have also been measured in conventional high energy ion implantation systems using a corner cup arrangement as disclosed in U.S. Pat. No. 4,751,393 issued Jun. 14, 1988 to Corey, Jr. et al. A mask having a central opening is positioned in the path of the ion beam. The beam is scanned over the area of the mask with the portion passing through the central opening impinging on the wafer. Small Faraday cups are located at the four corners of the mask and sense the beam current at these locations.
Accordingly, there is a need for improved methods and apparatus for measuring ion dose implanted into a workpiece in plasma doping systems.
According a first aspect of the invention, plasma doping apparatus is provided. The plasma doping apparatus comprises a plasma doping chamber, a platen mounted in the plasma doping chamber for supporting a workpiece, typically a semiconductor wafer, wherein the platen and the workpiece constitute a cathode, a source of ionizable gas coupled to the chamber, an anode spaced from the platen and a pulse source for applying high voltage pulses between the cathode and the anode. The high voltage pulses ,Produce a plasma having a plasma sheath in the vicinity of the workpiece. The plasma contains positive ions of the ionizable gas. The high voltage pulses accelerate the positive ions across the plasma sheath toward the platen for implantation into the workpiece. The plasma doping apparatus further comprises one or more Faraday cups positioned adjacent to the platen for collecting a sample of the positive ions accelerated across the plasma sheath. The sample is representative of the dose of positive ions implanted into the workpiece. The Faraday cup has a cover with a plurality of apertures. The ions collected by the Faraday cup pass through the apertures into an interior chamber of Faraday cup and are detected.
Preferably, each of the apertures has a width that is less than the thickness of the plasma sheath. The width of each aperture is selected to prevent formation of a discharge within the interior chamber of the Faraday cup.
In one embodiment, the cover comprises a multi-aperture plate. In another embodiment, the cover comprises a wire mesh. In a further embodiment, the cover of the Faraday cup may comprise a front conductor facing the plasma, a back conductor facing the interior chamber of the Faraday and an insulator separating the front conductor and the back conductor. The back conductor may be biased to repel electrons.
According to a second aspect of the invention, plasma doping apparatus is provided. The plasma doping apparatus comprises a plasma doping chamber, a platen mounted in the plasma doping chamber for supporting a workpiece, typically a semiconductor wafer, wherein the platen and the workpiece constitute a cathode, a source of ionizable gas coupled to the chamber, an anode spaced from the platen and a pulse source for applying voltage pulses between the cathode and the anode. The voltage pulses produce a plasma having a plasma sheath in the vicinity of the workpiece. The plasma contains positive ions of the ionizable gas. The voltage pulses accelerate the positive ions across the plasma sheath toward the platen for implantation into the workpiece. The plasma doping apparatus further comprises one or more Faraday cups positioned adjacent to the platen for collecting a sample of the positive ions accelerated across the plasma sheath. The sample is representative of the dose of positive ions implanted into the workpiece. The Faraday cup includes means for producing within an interior chamber of the Faraday cup an electric field lateral to the direction of ions entering the Faraday cup.
The means for producing electric fields may comprise an electrode located within the interior chamber of the Faraday cup and a supply voltage coupled to the electrode for biasing the electrode with respect to a wall of the Faraday cup. When the interior chamber of the Faraday cup is cylindrical, the electrode may comprise an axial conductor located within the interior chamber. When the Faraday cup has an annular configuration, the electrode may comprise an annular electrode located between inner and outer walls of the Faraday cup. In another embodiment, an annular Faraday cup may have electrically isolated inner and outer walls. A voltage may be applied between the inner and outer walls for producing within the interior chamber of the Faraday cup an electric field lateral to the direction of movement of the ions entering the Faraday cup.
The apparatus may include a single Faraday cup or two or more Faraday cups disposed around the platen. The plasma doping apparatus may include a guard ring, and the Faraday cup may be embedded within the guard ring. The guard ring can be maintained at either the cathode potential or at another potential which is selected to control the plasma uniformity.